The size of the reamer would be determined by the size and kind of the pins you are using.
I had to do that once, welded up the original holes, aligned the front sight and then re-drilled them using the front sight base as a guide....what a pain in the a** that was cause I didn't have a fixture specifically made for performing that operation.
If possible....I would send it back to the manufacturer and have them correct it, that is less of a hassle.

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